For a little while I didn’t have a car so I bought a Sparkel beverage machine, which is unique because instead of CO2 canisters it uses two powder packets that you mix together in a chamber. It adds water from a reservoir into the chamber and the resulting chemical reaction produces enough CO2 to circulate through a bottle and carbonate your drink.

And the proprietary powder packets are just citric acid and baking soda, so if you know what your doing (I’ve forgotten the measurements tbh) you can carbonate for real cheap. You could also carbonate liquids other than water with it and add things like ice or fruit or lemon slices before carbonating.

It’s more of a hassle than CO2 tanks though. It’s about as involved as a fancy coffee maker, and on rare occasions it would fail due to clogs. I was always able to get it running again and managed to make my money back on it from carbonating tap water. I went back to canned seltzer once I got a new car though.

Others have said it gives the seltzer off flavors as well but I never noticed it except for one time when I think I put too much of one powder in.